The presence of dissolved phosphate in industrial effluents and wastewater is a long-standing problem in the art. Phosphorus (referred to herein as “P”) is a non-renewable resource and an important non-substitutable macronutrient, existing in nature as phosphates in various inorganic or organic forms, and ranging from the simple to the very complex in terms of molecular structure. Because P is essential for all biological processes, there is concern that the current demand and exploitation (total annual production is about 20 million tons of P, derived from roughly 140 million tons of rock concentrates) of this non-renewable resource is not sustainable. Nearly all the P used globally is mined from a relatively small number of commercially-exploitable deposits, and it has been estimated that the global economic P reserves may last about 100 years at the current rate of extraction. Therefore, the world's P resources are finite and should be used efficiently and in a sustainable way. Additionally, aside from the non-renewable resource aspect, there is need to improve P management, particularly from the environment protection perspective because, for example, P-enrichment in receiving waters is associated with harmful algae blooms that affect the health and vitality of wetlands and marine environments. Therefore, there is a pronounced need in the art to develop methods for increasing the life expectancy of the world's limited P resources. There is a pronounced need in the art to develop methods for recovery and recycling of P from industrial effluents and wastewater.
Art-recognized P removal technologies applied to wastewater include chemical and biological processes. One chemical technology for P removal and recovery is crystallization of P in the form of struvite (magnesium ammonium phosphate hexahydrate or MgNH4PO4.6H2O). Struvite is crystalline and thus well suited for formation from effluent streams. In addition, as a granular product struvite is more compact than other chemical precipitates, and it performs well as a slow-release fertilizer. Struvite formation requires reaction between three soluble ions in solution, Mg2+, NH4+ and PO43−, to form precipitates with low solubility (struvite has a pKsp of 12.6). Struvite precipitation is controlled by pH, supersaturation, and presence of impurities, such as calcium. High pH (e.g., pH 8.5) and supersaturation of the three ions are favorable to struvite formation.
As part of secondary sewage treatment, primary treated sewage is treated with air or oxygen. In the activated sludge process, microorganisms utilize oxygen to metabolize the incoming waste sewage thereby forming a mixture of microorganisms and sewage (mixed liquor). This mixture is conducted to settling tanks for concentration to provide concentrated activated sludge. A majority of the sludge is returned to the activated sludge process and a separate portion of this sludge (waste activated sludge) is removed from the activated sludge process and conducted to a sludge handling system for further treatment and disposal.
In a typical wastewater treatment process, waste activated sludge is conducted to a first solids separation (centrifuge or other thickening apparatus) for thickening, where the liquids are tapped off and returned to the wastewater plant for treatment and the resultant thickened sludge is conducted to an anaerobic digester with other sludge where it remains for a period of time before being conducted to a second solids separation (centrifuge or other dewatering apparatus) for dewatering. Struvite tends to form in the digester and other downstream equipment because of the ammonia, magnesium and phosphorus present in the process streams. This struvite is impractical to harvest and also has the deleterious effect of being deposited on surfaces in the process system components.
Despite the advances in removing phosphorus from wastewater through the formation and recovery of struvite, a need exists for improved methods and systems for wastewater treatment and struvite recovery that eliminate or significantly reduce nuisance struvite formation. The present invention seeks to fulfill this need and provides further related advantages.